Over the past few decades, tremendous progresses have been made in the field of organic electronic devices, e.g. OLEDs (Organic Light Emitting Diodes), OTFTs (Organic Thin Film Transistors), OPVs (Organic Photovoltaics) and the like; especially for OLED materials, they have been recognized worldwide as being applicable to the next generation of novel flat-panel display technology due to their advantages, such as spontaneous optical rotation, high contrast, high response rate, wide angle of view, low power consumption, full color, simple fabrication process, etc. Thus in recent years, these OLED materials have drawn great attention from acedemic and industry.
Fast progresses have been made in the application and industrialization of OLED materials since Qingyun Deng and Van Slyke from Kodak began their groundbreaking work on OLED materials. Among those factors that affect the performances of OLED devices, the purity of OLED materials is one of the most important, which has a direct impact upon not only the performances but also the reliability of the devices. In fact, the purity of an OLED material has a direct impact upon the charge (electron and hole) transport capacity, so it is a key aspect to examine for satisfying performance of an organic electronic device. In terms of technology, OLED materials are currently divided into two main categories: small-molecular organic materials and high-molecular materials. Among the commercially available materials so far, the small-molecular OLED organic materials are employed by most of the companies. At present, there are several main purification processes for the acquisition of high-purity small-molecular organic materials, e.g. zonal melting process, chromatogram comparison process, in-situ filtration technique, temperature gradient sublimation separation process and so on. Among these processes, the temperature gradient sublimation separation technique is the most useful and common purification method for organic semiconductor small-molecular materials, because there is no liquid-phase state in most of the organic small-molecular substances at atmospheric pressure or low pressure. At present, this technique is being used by a large number of OLED material researchers and manufacturers for the production of highly pure organic small molecular OLED materials. The mechanism of this process relies on a temperature gradient, which is built up by stepwise heating of the sublimation tube, allowing the main product and impurities with different sublimability to condense seperately. This process is shown in FIG. 1.
During heating in the heating region 1 as shown in FIG. 1, the pure product is enriched on the inner wall of the tube in the collection region 2 (a heating region with a set particular temperature), in the presence or absence of an inert gas flow, while more volatile impurities are delivered and deposited in the impurity region 3, which has a certain distance away from the product, and the involatile impurities remain in the sample boat. This conventional approach is simple, however, a single time of sublimation typically fails to meet the purity requirement and the product needs to be sublimed for a second time or even more times to reach sufficiently high purity, thus the entire purification procedure is quite time-consuming and material-consuming. It is therefore necessary to improve the sublimation purification method for obtaining highly pure materials, and eventually high-efficiency and low-cost devices.